Field emission type electron microscope using a multi-stage acceleration tube

Abstract

A field emission type electron microscope using a multi-stage acceleration tube wherein an acceleration voltage to be applied to at least one, always inclusive of a first-stage acceleration electrode, of acceleration electrodes is changed in interlocked relationship with a change in a field emission voltage to be applied to a field emission electrode, so that power of an electrostatic lens can be kept constant.

We claim: 
     
       1. A field emission type electron microscope using a multi-stage acceleration tube, comprising: a field emission cathode;   a field emission electrode for causing field emission of electrons from said cathode, said field emission electrode having an aperture provided therein;   acceleration electrode means including a first-stage acceleration electrode and at least one succeeding-stage acceleration electrode for accelerating an electron beam passing through said aperture of said field emission electrode; and   application voltage interlocking means for changing, an acceleration voltage to be applied across said field emission cathode and at least one acceleration electrode of said acceleration electrode means inclusive of said first-stage acceleration electrode, in interlocked relationship with a change in a field emission voltage to be applied across said field emission cathode and said field emission electrode, said application voltage interlocking means interlocking said acceleration voltage with said field emission voltage so as to provide a ratio of said acceleration voltage to said field emission voltage in a range of 6 to 15.   
     
     
       2. The electron microscope according to claim 1, wherein said application voltage interlocking means comprises a first variable voltage source for supplying said field emission voltage and a second variable voltage source, for supplying said acceleration voltage. 
     
     
       3. The electron microscope according to claim 1, wherein said application voltage interlocking means supplies, as said field emission voltage and said acceleration voltage, divisional voltages generated by dividing a single power source by a series connection of resistors. 
     
     
       4. The electron microscope according to claim 1, wherein said first-stage acceleration electrode is shaped into a configuration which satisfies a relation of   4D.sub.1 ≦D.sub.2 ≦L     where D 1  is a minimum aperture size at the inlet of the electron beam, D 2  is a minimum aperture size at the outlet of the electron beam, and the distance as measured along the electron beam travelling direction between the inlet and outlet is L.   
     
     
       5. The electron microscope according to claim 1, wherein said application voltage interlocking means supplies to the respective acceleration electrodes of said acceleration electrode means divisional voltages generated by dividing a single acceleration power source by resistors whose resistances are variable. 
     
     
       6. The electron microscope according to claim 1, further comprising a first acceleration power source for supplying said acceleration voltage interlocked with said field emission voltage by means of said application voltage interlocking means, a second acceleration power source connected in series therewith, and means for negative feedback of the output of said first acceleration power source to said second acceleration power source.